The present invention relates generally to a real image type finder optical system and an imaging system comprising the same, and more particularly to a real image type finder optical system that is suitably used with lens shutter cameras, electronic still cameras, etc., in which a taking optical system is provided separately from a finder optical system.
Virtual image type finders and real image type finders are generally known for finders mounted on lens shutter cameras. To try to slim down cameras, however, the real image type finders are still used because the virtual image type finders incur an increase in lens diameters.
As, in recent years, camera makers have increasingly stressed size and cost reductions of cameras, further size and cost reductions have been demanded for finder optical systems mounted on the cameras. So far, zoom types such as two-group, three-group and four-group zoom types, wherein a prism is counted as a sort of lens group, have been proposed as optical systems for objective lenses. For a real image type zoom finder that has a high zoom ratio among these finders, it is desired in view of aberration correction that the associated objective optical system be constructed as a zoom type comprising four or more lens groups. However, the use of two or more lenses in one lens group is not preferred. The reason is that the amount of movement of moving lens groups during zooming is limited. This means that the power of each lens group must be increased, leading to noticeable performance degradation due to decentration errors. etc. during fabrication. To achieve significant size reductions and cost reductions, it is thus preferable to construct each lens group of the objective optical system using one lens (including a prism).
For some zoom types comprising four or more lens groups, it is desired to construct the first lens group using a negative lens, because when the first lens group is made up of a positive lens, the entrance pupil comes near to the primary image-formation plane side, resulting in an increase in the lens diameter. For size reductions, it is also preferable to construct the second, and the third lens group in the form of a positive lens group, because a long enough back focus is ensured with the first to third lens group, leading to an increase in the degree of freedom in prism construction. However, it is not preferable to construct the fourth lens group (prism) using a positive lens, because the lens diameter becomes too large. In other words, the fourth lens group should be made up of a negative lens having a small effective diameter. This also contributes to ensuring the degree of freedom in prism construction.
Such a finder that comprises four objective lens groups, each made up of one lens, and has a −++− construction as viewed in order from its object side, for instance, is set forth in the following patent publications 1 to 6.
Patent Publication 1
JP-A 5-164964 (Paragraphs 0011 to 0029, and FIGS. 1 to 15)
Patent Publication 2
JP-A 8-240769(Paragraphs 0022 to 0080, and FIGS. 1 to 13)
Patent Publication 3
JP-A 11-194271 (Paragraphs 0024 to 0070, and FIGS. 1 to 5)
Patent Publication 4
JP-A 11-242167 (Paragraphs 0041 to 0218, and FIGS. 1 to 18)
Patent Publication 5
JP-A 2000-347101 (Paragraphs 0015 to 0046, and FIGS. 1 to 12)
Patent Publication 6
JP-A 2001-133700 (Paragraphs 0008 to 0024, and FIGS. 1 to 16)